Generally speaking, an iris of the human eye is a coloured area encircling the pupil and surrounded by the sclera. The iris has several areas of different colours. The colour of a given area depends on the melanin density in this given area.
The greater the melanin density in this given area, the more this area will appear dark when observed in the visible light spectrum.
The quantity of melanin in a given area of the iris is liable to change over time as a function of parameters such as an individual's exposure to sunlight or an iris pigmentation disorder. This means that the colour data of the areas of the iris are inadequate to guarantee recognition/authentication/identification of a given iris of a given individual.
The iris comprises a large number of muscle fibres serving to dilate, or conversely, constrict the pupil. The majority of these fibres, approximately 6000, are arranged in radii around the pupil.
These fibres are more or less dense and more or less intertwined and form a network, the network of which is unique to each eye.
Iris recognition systems are known in applications for authenticating individuals. These systems are however susceptible to deception by presenting an image of an iris of the human eye or a dummy iris. In order to limit the risks of fraud against the iris recognition system, it has been contemplated to use an image of this iris in the visible range and an image of this same iris in the near infra-red range. Thus, document FR2864290_A1, of the same applicant, discloses a biometric detection method in which one generates:                a first image of an iris of the human eye observed using first means of image capture sensitive in a visible light spectrum; and        a second image of this iris observed using second means of image capture sensitive in an infra-red spectrum.        
The first colour image is used to detect the outer edge of the iris and facilitate localisation of a corresponding area of biometric interest located in the second image in the infra-red spectrum in order to seek biometric characteristics therein, in this instance textural characteristics. Authentication is performed in this case by seeking characteristics typical of the network or the texture of the iris in the area of interest of the second image in the infra-red spectrum. This document FR2864290_A1 also describes a type of attempted fraud against the system for recognising the iris of the human eye, performed by presenting an image of the iris in visible ink in the infra-red spectrum. Since this type of ink has, in the visible spectrum, a specific colour different from the colour of genuine irises, this document FR2864290 proposes a method of detecting attempt fraud by colorimetric analysis of the first image.
In order to improve security of the iris recognition system, it is useful to improve detection of instances of attempted fraud against the system.